Serum human chorionic gonadotropin measurement in the diagnosis of ectopic pregnancy when transvaginal sonography is inconclusive

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1 FERTILITY AND STERILITY VOL. 70, NO. 5, NOVEMBER 1998 Copyright 1998 American Society for Reproductive Medicine Published by Elsevier Science Inc. Printed on acid-free paper in U.S.A. Serum human chorionic gonadotropin measurement in the diagnosis of ectopic pregnancy when transvaginal sonography is inconclusive Ben W. J. Mol, M.D.,* Petra J. Hajenius, M.D., Ph.D., Simone Engelsbel, M.D., Willem M. Ankum, M.D., Ph.D., Fulco Van der Veen, M.D., Ph.D., Douwe J. Hemrika, M.D., Ph.D., and Patrick M. M. Bossuyt, Ph.D.* Academic Medical Center, University of Amsterdam, and Onze Lieve Vrouwe Gasthuis, Amsterdam, the Netherlands Received January 30, 1998; revised and accepted May 29, Supported by grant OG 93/007 from the Dutch Health Insurance Council, Amstelveen, the Netherlands. Reprint requests: Ben W. J. Mol, M.D., Department of Clinical Epidemiology and Biostatistics, Academic Medical Center, University of Amsterdam, P.O. Box 22700, 1100 DE Amsterdam, the Netherlands (FAX: ; * Department of Clinical Epidemiology and Biostatistics, University of Amsterdam. Department of Obstetrics and Gynecology, University of Amsterdam. Department of Obstetrics and Gynecology, Onze Lieve Vrouwe Gasthuis. Center for Reproductive Medicine, University of Amsterdam /98/$19.00 PII S (98) Objective: To assess the accuracy of initial and repeated serum hcg measurements in the diagnosis of ectopic pregnancy (EP) in patients in whom transvaginal sonography is inconclusive and to evaluate whether patient characteristics influence the accuracy of serum hcg measurements. Design: Prospective study. Setting: Two large teaching hospitals in Amsterdam, the Netherlands. Patient(s): Three hundred fifty-four consecutively seen pregnant patients with suspected EP and inconclusive transvaginal sonographic findings. Intervention(s): Serum hcg measurements. Main Outcome Measure(s): The performance of repeated serum hcg measurements in the diagnosis of EP was evaluated through the analysis of receiver operating characteristic curves. Result(s): Initial serum hcg measurements were more diagnostic in conjunction with sonographic evidence of an ectopic mass or fluid in the pouch of Douglas than in the absence of sonographic abnormalities. On repeated measurement, the course of the serum hcg concentration provided more diagnostic information than did the absolute serum hcg concentration 2 and 4 days after the start of the diagnostic process. Conclusion(s): The interpretation of serum hcg measurements should depend on additional findings at transvaginal sonography. A cutoff level of 1,500 IU/L is recommended for patients with an ectopic mass or fluid in the pouch of Douglas; in patients without these findings, the cutoff level should be at least 2,000 IU/L. Four days after the start of the diagnostic process, any rise in the serum hcg concentration makes the diagnosis of EP very likely. (Fertil Steril 1998;70: by American Society for Reproductive Medicine.) Key Words: Diagnosis, ectopic pregnancy, serum human chorionic gonadotropin, transvaginal sonography Transvaginal sonography and serum hcg measurements have proven to be reliable tools in the noninvasive diagnosis of ectopic pregnancy (EP) (1). Transvaginal sonography can discriminate between intrauterine pregnancy (IUP) and EP by visualizing the uterine cavity and the adnexal region, respectively. The presence of an IUP virtually rules out the possibility of an EP. The presence of a gestational sac with a yolk sac and/or cardiac activity in the adnexal region virtually proves the presence of an EP, justifying the start of treatment. The presence of an ectopic mass or fluid in the pouch of Douglas increases the likelihood of an EP but does not justify the immediate start of treatment. Transvaginal sonography also is inconclusive when no abnormalities are found in the adnexal region. Serum hcg measurements then may be used as an additional diagnostic tool in these patients (2 6). The main concern in the noninvasive management of EP is to make sure that each patient in whom the diagnosis is made really has an EP, preventing unnecessary or even harmful treatment. It is less important to identify immediately those patients who do not have an EP, because sonography and serum hcg measurements can be repeated until a diagnosis is 972

2 made with certainty. Consequently, diagnostic tests for EP should combine an almost perfect specificity with the best possible sensitivity. Optimization of the cutoff value therefore should focus on the serum hcg concentration above which failure to image a gestational sac could be taken as a presumptive index for the presence of an EP. In 1981, Kadar et al. (2) proposed the use of a discriminatory zone for serum hcg concentrations in the diagnosis of EP. The discriminatory hcg zone was defined as the minimal hcg concentration above which the sac of an IUP always can be identified by sonography. Consequently, the diagnosis of an EP can be made when an intrauterine gestational sac is absent and the serum hcg concentration is above the discriminatory zone. At its introduction, sonography was performed transabdominally and the discriminatory zone was situated between 6,000 IU/L and 6,500 IU/L (2). Since then, the resolution of sonography has improved greatly, and the discriminatory zone has decreased. Studies reporting on the use of serum hcg measurements combined with sonography in the diagnosis of EP have recommended cutoff levels varying between 1,000 IU/L and 6,500 IU/L (2 6). Thus far, no studies have used analysis of receiver operating characteristic (ROC) curves to identify the optimal cutoff value for serum hcg measurements. A diagnosis cannot be established immediately for patients with an inconclusive sonogram and a low serum hcg concentration. Applying a 2-day interval between repeated hcg measures, the use of a cutoff value for the serum hcg concentration of 1,000 IU/L (Third International Standard) leads to a sensitivity of 90% and a specificity of 98% in the diagnosis of EP (7, 8). Others have reported that in the interpretation of repeated serum hcg measurements, the course of the serum hcg concentration provides more diagnostic information than does the absolute serum hcg concentration in the identification of a viable IUP (9 12). This issue has become of interest because recent data have indicated that systemic methotrexate is an effective therapy for EP in patients with serum hcg concentrations of 3,000 IU/L (13, 14). It is therefore important to rule out a viable IUP with certainty before starting methotrexate treatment. The first aim of this prospective study was to assess the discriminative capacity of serum hcg measurements through the analysis of ROC curves. The impact of different patient characteristics on the discriminative capacity of serum hcg measurements also was assessed. The second aim was to compare the accuracy of the absolute value of serum hcg measurements and the course of serum hcg concentrations in the diagnosis of EP and of viable IUP in patients in whom initial serum hcg measurements do not reveal an immediate diagnosis. MATERIALS AND METHODS Consecutively seen pregnant patients with clinically suspected EP in the Academic Medical Center and the Onze Lieve Vrouwe Gasthuis between September 1993 and April 1996 were entered into a prospective study. Both centers are large teaching hospitals that serve a population of mixed ethnicity in Amsterdam, the Netherlands. The study was approved by the institutional review boards of both hospitals. Pregnancy had to be confirmed by a urine pregnancy test (Icon-test; Hybritech Inc., San Diego, CA). Ectopic pregnancy was suspected in patients who had one or more of the following inclusion criteria: [1] clinical symptoms (abdominal pain and/or vaginal bleeding); [2] the presence of one or more risk indicators for EP; [3] routine sonography, performed after a gestational age of 6 weeks, that failed to show an intrauterine gestational sac; and/or [4] the microscopic absence of chorionic villi after dilatation and curettage (6). The following were considered risk indicators: a previous EP, known tubal pathology detected on hysterosalpingography and/or laparoscopy, previous tubal surgery, pelvic inflammatory disease, diethylstilbestrol exposure in utero, and sterilization or a contraceptive device in situ at the moment of conception. Hemodynamically unstable patients were excluded from the study. After patients gave informed consent, transvaginal sonography was performed (Hitachi EUB 415/515; Hitachi Medical Corporation, Tokyo, Japan). Sonography was performed by one of the study investigators or, during shifts, by the resident on call. The intrauterine cavity was scanned and an IUP was diagnosed when an intrauterine gestational sac was visualized. When an intrauterine gestational sac could not be visualized, both adnexal regions were scanned for the presence of an ectopic gestational sac, an ectopic mass, or fluid in the pouch of Douglas. An ectopic gestational sac was defined as the presence of a yolk sac, a fetal pole, or fetal cardiac activity. When an ectopic gestational sac was visualized, an EP was diagnosed. After sonography had been performed, the serum hcg concentration was determined with the use of the Microparticle Enzyme Immunoassay (Imx analyzer; Abbott Diagnostics Division, Chicago, IL) and expressed in international units per liter. An EP was diagnosed when the serum hcg concentration was 1,500 IU/L in patients in whom sonography failed to show an intrauterine or ectopic gestational sac. An exception was made for patients who had a clinical picture suggestive of a complete miscarriage. Those patients were managed expectantly and excluded from the study. In cases where transvaginal sonography showed no gestational sac but the serum hcg concentration was 1,500 IU/L, the patients were reevaluated 2 days later on an outpatient basis. If, at reevaluation, a pregnancy was detected on transvaginal sonography within or outside the uterine cavity, a diagnosis of a viable IUP or an EP was made, respectively. FERTILITY & STERILITY 973

3 In cases where sonographic findings were repeatedly negative, further management depended on the repeated serum hcg concentrations. Serum hcg concentrations of 1,000 IU/L that were obtained 2 4 days after the start of the diagnostic process were assumed to be conclusive for an EP. When three consecutive serum hcg concentrations were 1,000 IU/L and sonographic findings were repeatedly negative, the diagnosis of a nonviable pregnancy was made (6). However, if a plateauing serum hcg pattern developed, which was defined as a rise in two consecutive measurements or no decline in three consecutive measurements, the diagnosis of an EP was made. The diagnosis was verified in several ways. The diagnosis of an EP was verified by laparoscopy. The diagnosis of an IUP was verified by repeated sonography at a gestational age of 12 weeks or by histopathologic evaluation in case of miscarriage; when the serum hcg concentration declined, it was measured repeatedly until it declined below the detection threshold (6). Thus, the three final diagnostic categories were EP, viable IUP, and a rest category of nonviable pregnancy that included nonviable IUPs and chemical pregnancies that resolved without treatment. The analysis was limited to those patients in whom transvaginal sonographic findings were inconclusive (i.e., to those patients in whom a gestational sac could not be visualized either within or outside the uterine cavity). Thus, patients in whom transvaginal sonography had demonstrated a gestational sac within or outside the uterine cavity were excluded from the analysis because sonography had resolved the diagnostic problem. Further, patients in whom pregnancy was a result of IVF-ET were excluded because the transfer of multiple embryos could influence the cutoff levels for test result positivity (15). Patients with missing data also were excluded. Likelihood ratios and their 95% confidence intervals (CIs) for each category of sonographic findings were calculated. An ROC curve was constructed, illustrating the capacity of serum hcg measurements to diagnose EP, and the area under the ROC curve was calculated. The area under the ROC curve expresses the performance of a diagnostic test taking values in the range of An area under the ROC curve of 0.5 implies that the diagnostic test under study has a discriminative capacity that does not exceed chance, whereas an area under the ROC curve of 1 implies that the discriminative capacity of the test under study is perfect. Subsequently, the diagnostic accuracy of serum hcg measurements in association with patient characteristics was evaluated. We compared the distribution of serum hcg concentrations in subgroups of patients with the use of the Kolmogorov-Smirnov test (16). Subgroups were defined based on the presence or absence of abdominal pain, vaginal bleeding, and an ectopic mass and/or fluid in the pouch of Douglas at transvaginal sonography. In all comparisons, a P value of.05 was considered to indicate a statistically significant difference between the distributions. Whenever distributions of serum hcg concentrations were found to be significantly different between patients with and without any of the studied characteristics, these distributions were plotted to visualize the effect of the characteristics on diagnostic accuracy. Such differences could have an impact on the optimal cutoff concentration and therefore could be of clinical significance. When the scatter plot indicated clinical significance, analysis of ROC curves was performed in the subgroups of patients with and without the specified characteristic, and likelihood ratios were calculated for various levels of serum hcg concentrations. For patients with serum hcg concentrations of 1,500 IU/L, the serum hcg concentrations obtained 2 days and 4 days after the start of the diagnostic process were compared with the final diagnosis of EP, viable IUP, or nonviable pregnancy. To evaluate the diagnostic accuracy of these repeated serum hcg measurements, ROC curves of the absolute serum hcg concentration, the absolute difference, and the percent difference were constructed for the diagnosis of EP and the diagnosis of viable IUP. These curves were compared for statistically significant differences (17). Likelihood ratios were calculated of the absolute serum hcg concentration at repeated measurement and of the absolute and percent difference between the initial and repeated serum hcg measurements. We calculated likelihood ratios for the presence of EP and for the presence of viable IUP. RESULTS During the study period, 824 consecutively seen patients presented with suspected EP. Twenty-three hemodynamically unstable patients were excluded. Transvaginal sonography revealed an intrauterine gestational sac in 333 patients, and the diagnosis of IUP was made. The IUP was confirmed for 323 patients, whereas 10 patients had an EP. Transvaginal sonography revealed an ectopic gestational sac in 74 patients. An EP was confirmed at laparoscopy in 72 patients (2 patients did not undergo laparoscopy because their serum hcg concentrations were 200 IU/L). Ten patients presented with a clinical picture of complete miscarriage and also were excluded. Among the remaining 384 women, pregnancy resulted from IVF-ET in 26. The serum hcg concentrations were not available for 4 women, all of whom were determined to have an EP. Among the 354 included patients, 223 had abdominal pain, 228 had vaginal bleeding, and 134 had at least one risk indicator for EP. In 34 patients, no intrauterine gestational sac could be detected on routine sonography performed after a gestational age of 6 weeks, and in 14 patients, histopathologic evaluation showed no chorionic villi in specimens obtained at curettage. An ectopic mass was detected at initial sonography in 58 patients. Fourteen patients showed fluid in 974 Mol et al. Techniques and instrumentation Vol. 70, No. 5, November 1998

4 FIGURE 1 Receiver operating characteristic curve of initial serum hcg measurements in the diagnosis of ectopic pregnancy. the pouch of Douglas on sonography. Twenty patients had an ectopic mass and fluid in the pouch of Douglas. An EP was verified in 129 (36%) of the 354 patients, whereas a viable IUP was sonographically verified in 67 (19%) patients. The other 158 patients had a nonviable pregnancy; a chemical pregnancy with declining serum hcg concentrations was verified in 135 (38%) patients, whereas 23 (6%) patients appeared to have had a miscarriage. The likelihood ratios for the presence of EP were 3.6 (95% CI, ) for an ectopic mass without fluid, 4.4 (95% CI, ) for the presence of only fluid in the pouch of Douglas, 9.9 (95% CI, 3 33) for the presence of an ectopic mass and fluid in the pouch of Douglas, and 0.55 (95% CI, ) for the absence of adnexal abnormalities. Figure 1 shows the ROC curve for initial serum hcg measurements in the diagnosis of EP. The area under the ROC curve was 0.81 (95% CI, ). In 7 of 76 patients with an initial serum hcg concentration of 1,500 IU/L, EP was not confirmed at laparoscopy, whereas in 2 patients with a serum hcg concentration of 1,500 IU/L, laparoscopy was delayed. Of these 9 false-positive findings, 1 patient had an ectopic mass at sonography and the other 8 had no findings whatsoever at initial sonography (Table 1). The final diagnosis in these patients was molar pregnancy, triplet pregnancy, miscarriage (n 2), chemical pregnancy (n 3), and nonviable IUP (n 2), respectively. The Kolmogorov-Smirnov test statistic indicated that the distribution of initial serum hcg concentrations in patients with an EP differed significantly between patients in whom transvaginal sonography showed either an ectopic mass or fluid in the pouch of Douglas and patients who had no sonographic abnormalities (P.01). The median serum hcg concentration was 1,320 IU/L (range, 85 42,000 IU/L) in patients with additional sonographic findings and 668 IU/L (range, 68 84,000 IU/L) in patients without additional sonographic findings. No statistically significant differences were found between the serum hcg distributions of patients with and without abdominal pain (P.17) or between the serum hcg distributions of patients with and without vaginal bleeding (P.85). The distribution of initial serum hcg measurements in patients without an EP did not differ significantly between TABLE 1 Distribution of initial serum hcg concentrations in patients without a viable pregnancy at transvaginal sonography, stratified for patients with and without an ectopic mass or fluid in the pouch of Douglas at sonography. Initial serum hcg concentration (IU/L) who had an EP who had a viable IUP who had a nonviable pregnancy LR for EP 95% CI LR for IUP 95% CI Patients with an ectopic mass or fluid in the pouch of Douglas at sonography n 66 n 1 n 25 1, ,000 hcg 1, ,500 hcg 2, , Patients without an ectopic mass or fluid in the pouch of Douglas at sonography n 63 n 66 n 133 1, ,000 hcg 1, ,500 hcg 2, , Note: LR likelihood ratio; CI confidence interval. FERTILITY & STERILITY 975

5 FIGURE 2 (A), Distribution of serum hcg concentrations in patients with and without an ectopic pregnancy, stratified for the presence or absence of an ectopic mass or fluid in the pouch of Douglas at transvaginal sonography (TVS). (B), Corresponding ROC curve after stratification for additional findings at transvaginal sonography. patients in whom transvaginal sonography showed an ectopic mass or fluid in the pouch of Douglas and patients who had no sonographic abnormalities in the adnexal region (P.06), between patients with and without abdominal pain (P.37), and between patients with and without vaginal bleeding (P.06). Figure 2A shows the distribution of serum hcg concentrations in patients with and without an EP, stratified for the presence of additional findings at transvaginal sonography. Among 262 patients who did not have an ectopic mass or fluid in the pouch of Douglas at transvaginal sonography, 63 (24%) had an EP. Among 92 patients who had either an ectopic mass or fluid in the pouch of Douglas at sonography, 66 (72%) had an EP. The corresponding ROC curves for hcg are plotted in Figure 2B. The area under the ROC curve was 0.74 (95% CI, ) for patients without an ectopic mass or fluid in the pouch of Douglas and 0.85 (95% CI, ) for patients with an ectopic mass or fluid in the pouch of Douglas. Table 1 shows the distribution of serum hcg concentrations in patients with and without an EP, taking into account additional sonographic findings. For patients in whom sonography showed an ectopic mass or fluid in the pouch of Douglas, the likelihood ratio increased strongly when the serum hcg concentration was 1,500 IU/L. For patients without additional findings, the likelihood ratio increased strongly when the serum hcg concentration was 2,000 IU/L. There were no viable IUPs when the serum hcg concentration was 1,500 IU/L. Two hundred eighty-five patients underwent reevaluation 2 days after the start of the diagnostic process. Transvaginal sonography led to a diagnosis of an EP in 11 patients and of a viable IUP in 52 patients. In the remaining 195 patients, the serum hcg concentration was used to establish a diagnosis. Of these patients, 38 (19%) eventually were found to have an EP, 15 (8%) a viable IUP, 16 (8%) a nonviable IUP, and 126 (65%) a chemical pregnancy. Two (5%) of the 38 EPs were complicated by tubal rupture and intraabdominal bleeding, diagnosed 4 days and 7 days after the start of the diagnostic process, respectively. The distributions of the first repeated serum hcg measurements in patients with an EP, a viable IUP, and a nonviable pregnancy and the corresponding likelihood ratios are shown in Table 2. Of seven patients who had a serum hcg concentration between 1,000 IU/L and 1,500 IU/L, three had an EP, two had a viable IUP, and two had a nonviable pregnancy. Five patients who had a serum hcg concentration at the first repeated measurement of 1,500 IU/L all were found to have an EP. 976 Mol et al. Techniques and instrumentation Vol. 70, No. 5, November 1998

6 TABLE 2 Distribution of absolute serum hcg concentrations at the first repeated measurement 2 days after the start of the diagnostic process, as well as the absolute and percent difference in serum hcg concentrations in patients in whom initial transvaginal sonography and serum hcg measurements revealed no diagnosis, and in whom repeated transvaginal sonography showed no pregnancy. Variable who had an EP (n 38) who had a viable IUP (n 15) who had a nonviable pregnancy (n 142) LR for EP 95% CI LR for IUP 95% CI Absolute serum hcg level (IU/L) hcg 1, ,000 hcg 1, ,500 hcg 2, , Absolute difference in serum hcg level (IU/L) Decline Rise Rise 501 1, Rise 1, Percent difference in serum hcg level Decline 50% Decline 50% Rise 50% Rise 50% Note: LR likelihood ratio; CI confidence interval. The distributions of the absolute and percent differences between the initial hcg measurement and the first repeated hcg measurement and the likelihood ratios also are shown in Table 2. A decline in the serum hcg concentration ruled out the possibility of a viable IUP. A decline of 50% also ruled out the possibility of an EP. A rise in the serum hcg concentration of 50% made the presence of an EP more likely (likelihood ratio 3.4). None of these patients had a viable IUP (likelihood ratio 0). A rise in the serum hcg concentration of 50% increased the risk of an EP (likelihood ratio 3.2), but increased more strongly the probability of a viable IUP (likelihood ratio 9.8). The ROC curves for the diagnosis of EP are shown in Figure 3A. The areas under the curve were 0.75 (95% CI, ) for the absolute serum hcg concentration, 0.81 (95% CI, ) for the absolute difference, and 0.83 (95% CI, ) for the percent difference. Both the absolute and the percent difference in the serum hcg concentration performed significantly better than did the absolute serum hcg concentration alone. (Overall P value.0003, absolute value versus absolute difference P value.0003, absolute value versus percent difference P value.0001, and absolute difference versus percent difference P value.39). The ROC curves for the diagnosis of viable IUP are shown in Figure 4. The areas under the curve were 0.69 (95% CI, ) for the absolute serum hcg concentration, 0.93 (95% CI, ) for the absolute difference, and 0.98 (95% CI, ) for the percent difference. The differences between the three ROC curves for the serum hcg concentration were statistically significant (P.0002). The absolute difference as well as the percent difference performed significantly better than did the absolute serum hcg concentration (P values.0001 and.0002, respectively). In addition, the percent difference performed significantly better than did the absolute difference (P.05). One hundred thirty-six patients underwent a second reevaluation 4 days after the start of the diagnostic process. Forty-six patients did not undergo a second reevaluation because their serum hcg concentration already was decreasing dramatically. Repeated transvaginal sonography led to a diagnosis in 41 patients: 17 EPs and 24 IUPs. Of the 24 IUPs, 14 were vital. In the remaining 95 patients, the serum hcg concentration was determined to establish a diagnosis. The distributions of the second repeated serum hcg measurements are shown in Table 2. Twenty-one (22%) patients had an EP, 1 patient had a viable IUP, and 73 (77%) had a nonviable pregnancy. The corresponding likelihood ratios of ranges of hcg concentrations for the presence of an EP are shown in Table 3. All 4 patients who had a serum hcg concentration of 1,000 IU/L had an EP. The presence of an EP was unlikely among the 60 patients who had a decline in the serum hcg concen- FERTILITY & STERILITY 977

7 FIGURE 3 (A), Receiver operating characteristic curves of absolute serum hcg level, absolute difference in serum hcg level, and percent difference in serum hcg level in the diagnosis of ectopic pregnancy 2 days after the start of the diagnostic process. (B), Receiver operating characteristic curves of absolute serum hcg level, absolute difference in serum hcg level, and percent difference in serum hcg level in the diagnosis of ectopic pregnancy 4 days after the start of the diagnostic process. tration of 50% (likelihood ratio 0.12). Any rise in the serum hcg concentration made the diagnosis of an EP very likely, with likelihood ratios of 10. However, 1 patient with a rising serum hcg concentration was found to have a viable IUP. Figure 3B shows the corresponding ROC curves. The areas under the ROC curve were 0.82 (95% CI, ) for the absolute serum hcg concentration, 0.89 (95% CI, ) for the absolute difference, and 0.91 (95% CI, ) for the percent difference. The ROC curves of the serum hcg concentration, the absolute difference, and the percent difference differed significantly (P.009), which was caused by a significantly better performance of the absolute and percent difference in serum hcg concentration compared with the absolute concentration alone (P.02 for both). The ROC curves of the absolute difference and the percent difference did not differ significantly (P.19). The construction of ROC curves for the diagnosis of the absence of a viable IUP was not possible because only one patient had a viable IUP. The number of patients who had repeated serum hcg measurements was too small to evaluate whether the diagnostic accuracy of serum hcg measurements was associated with patient characteristics such as abdominal pain, vaginal bleeding, and the presence of fluid in the pouch of Douglas, as was done for initial serum hcg measurements. DISCUSSION Since the first publication of the discriminatory hcg zone concept, the resolution of transvaginal sonography gradually has improved. Several studies have proposed lower cutoff values for serum hcg concentrations than the initial threshold of 6,000 IU/L as proposed by Kadar et al. in 1981 (2, 3, 5, 6). Some have used repeated measurements (5) and others have not focused on almost perfect specificity (3). In our earlier smaller study, we did not analyze ROC curves (6). In the present study, analysis of ROC curves showed that a serum hcg concentration of 2,000 IU/L almost certainly rules in EP. However, the accuracy of serum hcg measurements depended on the presence of additional findings at transvaginal sonography. In patients in whom sonography showed an ectopic mass and/or fluid in the pouch of Douglas, a serum hcg concentration of 1,500 IU/L indicated EP with virtual certainty. In the cohort under study, a cutoff value of 2,000 IU/L for patients without any sonographic abnormalities would have prevented five unnecessary laparoscopies, whereas the diagnosis of EP would have been delayed in one patient. The analysis of patients in whom serum hcg measurements did not produce an immediate diagnosis shows that with repeated serum hcg measurements, the course of serum hcg concentrations, be it the absolute or the percent 978 Mol et al. Techniques and instrumentation Vol. 70, No. 5, November 1998

8 FIGURE 4 Receiver operating characteristic curve of absolute serum hcg level, absolute difference in serum hcg level, and percent difference in serum hcg level in the diagnosis of viable intrauterine pregnancy 4 days after the start of the diagnostic process. difference in the serum hcg concentration, provides more diagnostic information than does the absolute serum hcg concentration alone. At the second repeated measurement, 4 days after the start of the diagnostic process, the serum hcg course was more accurate than at the first repeated measurement 2 days after the start of the diagnostic process; no tubal rupture occurred during this period. Verification bias, which occurs when verification of the diagnosis depends on the test under study, is a problem in the interpretation of our findings (18). Confirmative laparoscopy was performed when the serum hcg concentration initially was 1,500 IU/L or was 1,000 IU/L at repeated measurement, or when it reached a plateau after three consecutive measurements. As a consequence, in patients with such serum hcg concentrations, EP always was detected and treated, whereas in patients with lower serum hcg concentrations, chemical pregnancies were diagnosed with declining serum hcg levels and the patients were managed expectantly. Therefore, our study design may have overestimated the specificity of serum hcg measurements, because positive results of serum hcg measurement were followed by confirmative laparoscopy and subsequent treatment, when some of these patients could have been managed expectantly. However, expectant management in patients with such serum hcg concentrations would have been unethical in our opinion. Recently, Korhonen et al. (19) reported that expectant management was successful in 77% of patients with a suspected EP who had a serum hcg concentration of 5,000 IU/L. Unfortunately, these investigators did not evaluate the relation between serum hcg concentrations and failure of expectant management. Future studies are needed to determine the serum hcg concentration at which expectant management is safe. In the literature, the performance of a diagnostic test commonly is reported in terms of sensitivity, specificity, and likelihood ratios. When such parameters are used to evaluate the performance of a diagnostic test, the crucial underlying assumption is that these indices remain constant for patients with different clinical characteristics (20, 21). This assumption has been found to be erroneous in several clinical situations in which it has been tested, the best documented example being exercise tests in the diagnosis of ischemic heart disease (22 25). The present study shows that the patient characteristics of abdominal pain and vaginal bleeding had no influence on the diagnostic performance of serum hcg measurements. In contrast, sonographic detection of an ectopic mass and/or fluid in the pouch of Douglas had a strong impact on the diagnostic performance of serum hcg measurements. In fact, the presence of sonographic abnormalities makes the presence of an ectopically nidated pregnancy very likely. The serum hcg concentration in these patients is used as a prognostic indicator of the need for treatment, with 1,500 IU/L as an initial cutoff level. Because, in our opinion, a diagnosis is only useful when it results in a decision regarding therapy, we decided to classify patients with low serum hcg levels who might have an ectopically nidated pregnancy in the rest category of nonviable pregnancy. In the absence of sonographic abnormalities, the location of the pregnancy (i.e., intrauterine or ectopic) is really unknown, and the serum hcg levels can be interpreted according to the discriminatory zone principle (2). Unfortunately, the diagnostic performance of serum hcg measurements alone was significantly less than in the presence of such findings. This is even more of a problem when one takes into account the fact that the prevalence of EP differs strongly between patients with additional sonographic findings and patients without such findings. In patients with additional sonographic findings, the prevalence of EP (i.e., the pretest probability) was 72% (66/92). In contrast, in absence of additional sonographic findings, the prevalence was only 24% (63/162). Consequently, serum hcg concentrations should be interpreted with more care in the absence of additional sonographic findings. In fact, the performance of serum hcg measurements in the absence of these findings should be higher to compensate for the lower FERTILITY & STERILITY 979

9 TABLE 3 Distribution of absolute serum hcg concentrations at the second repeated measurement 4 days after the start of the diagnostic process, as well as the absolute and percent difference in serum hcg concentrations in patients in whom initial transvaginal sonography and serum hcg measurements revealed no diagnosis, and in whom repeated transvaginal sonography showed no pregnancy. Variable who had an EP (n 21) who had a viable IUP (n 1) who had a nonviable pregnancy (n 73) LR for EP 95% CI Absolute serum hcg level (IU/L) hcg 1, , Absolute difference in serum hcg level (IU/L) Decline Rise Rise Percent difference in serum hcg level (IU/L) Decline 50% Decline 50% Rise 50% Rise 50% Note: LR likelihood ratio; CI confidence interval. pretest probability. Unfortunately, in this case, the diagnostic performance of serum hcg measurements is lower. A cutoff level of 1,000 IU/L at the first repeated serum hcg measurements 2 days after the start of the diagnostic process, as used in this study, generated false-positive diagnoses. The analysis of the absolute serum hcg concentration showed that the cutoff levels at which an EP can be diagnosed specifically 2 days and 4 days after the start of the diagnostic process are 1,500 IU/L and 1,000 IU/L, respectively. However, the sensitivity at these cutoff levels is low (12% and 18%, respectively). Consequently, the discriminatory zone principle that has been introduced for the interpretation of initial serum hcg measurements is of limited value in the interpretation of repeated serum hcg measurements. Although it remains uncertain what would have happened with the eight EPs that were detected 2 days after the start of the diagnostic process if the diagnosis had been delayed, we conclude that, according to the results of our study, repeated serum hcg measurements can be delayed safely until 4 days after the start of the diagnostic process. Then, a decline in the serum hcg concentration of 50% virtually rules out an EP and these patients do not require frequent monitoring. A decline of 50% increases the probability of an EP and these patients should be monitored frequently. In patients who have a rise in the serum hcg concentration, the diagnosis of an EP becomes very likely. However, if the increase in these patients is 50%, a viable IUP cannot be ruled out and treatment with methotrexate or dilatation and curettage must be applied with great care. In conclusion, we recommend that in patients with a suspected EP in whom sonography does not reveal a clear diagnosis, the presence of sonographic abnormalities should be taken into account in the interpretation of serum hcg levels. A cutoff value for serum hcg of 1,500 IU/L seems appropriate for patients with an ectopic mass or fluid in the pouch of Douglas at sonography, whereas this cutoff level should be at least 2,000 IU/L for patients without these sonographic findings. On repeated hcg measurements, the course of the serum hcg concentrations provides more diagnostic information than does the absolute serum hcg concentration alone. Four days after the start of the diagnostic process, any rise in the serum hcg concentration makes the diagnosis of EP very likely. References 1. Carson SA, Buster JE. Ectopic pregnancy. N Engl J Med 1993;329: Kadar N, DeVore G, Romero R. Discriminatory hcg zone: its use in the sonographic evaluation for ectopic pregnancy. Obstet Gynecol 1981;58: Nyberg DA, Mack LA, Laing FC, Jeffreu RB. Early pregnancy complications: endovaginal sonographic findings correlated with human chorionic gonadotrophin levels. Radiology 1988;167: Romero R, Kadar N, Jeanty P, Copel J, Chervenak FA, DeCherney A, et al. Diagnosis of ectopic pregnancy: value of the discriminatory human chorionic gonadotrophin zone. Obstet Gynecol 1985;66: Cacciatore B, Stenman UH, Ylostalo P. Diagnosis of ectopic pregnancy by vaginal ultrasonography with a discriminative serum hcg level of 1,000 IU/L. Br J Obstet Gynaecol 1990;97: Ankum WM, Van der Veen F, Hamerlynck JVThH, Lammes FB. Laparoscopy: a dispensable tool in the diagnosis of ectopic pregnancy? Hum Reprod 1993;8: Hajenius PJ, Mol BWJ, Ankum WM, Van der Veen F, Bossuyt PMM, Lammes FB. Suspected ectopic pregnancy: expectant management in patients with negative sonographic findings and low serum hcg concentrations. Early Pregnancy 1995;1: Mol et al. Techniques and instrumentation Vol. 70, No. 5, November 1998

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